Ground state

physics

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Four-level laser

A sustained laser beam can be achieved by using atoms that have two relatively stable levels between their ground state and a higher-energy excited state. As in a three-level laser, the atoms first drop to a long-lived metastable state where they can be stimulated to emit excess energy. However, instead of dropping to the ground state, they stop at another state above the ground state from which they can more easily be excited back up to the higher metastable state, thereby maintaining the population inversion needed for continuous laser operation.

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Three-level laser

A burst of energy excites electrons in more than half of the atoms from their ground state to a higher state, creating a population inversion. The electrons then drop into a long-lived state with slightly less energy, where they can be stimulated to quickly shed excess energy as a laser burst, returning the electrons to a stable ground state.

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Figure 12: The energy levels of three nuclei with A = 6, relative to the ground state of 6Li after subtraction of the calculated electrostatic energy. The isospin T = 1 levels exist in all three nuclei.

atomic energy level

Atomic energy levels are typically measured by observing transitions between two levels. For example, an atom in its lowest possible energy state (called the
ground state) can be excited to a higher state only if energy is added by an amount that is equal to the difference between the two levels. Thus, by measuring the energy of the radiation that has been absorbed by the atom, the difference...

...= +
1/2 or -
1/2 state; these numbers specify the resulting configuration, or arrangement, of electrons of a hydrogen atom in its
ground state. If a positive charge is added to the nucleus along with a second external electron, the second electron will occupy the lowest energy state, again
n = 1,
l = 0,...

carbene bonding

The theory of chemical bonding predicts two fundamentally different electron configurations for carbenes, either one of which may correspond to the
ground state of the molecules (state of lower energy content) depending only on the nature of the atoms and groups attached to the divalent carbon atom. This duality arises from the fact that the two bonds of the carbene utilize only two of the four...

carbon group elements

The ground-state electronic configurations of atoms of these carbon group elements show that each has four electrons in its outermost shells. As has been explained, if
n represents the outermost shell (
n being two for carbon, three for silicon, etc.), then these four electrons are represented by the symbols
ns2np2. Such a...

definition

...are also called energy levels; the atom, or system, is said to undergo a transition between two energy levels when it emits or absorbs energy. The lowest energy level of a system is called its
ground state; higher energy levels are called excited states.
See also Franck-Hertz experiment.

ionization process

...the study of resonance ionization. Unless an atom is subjected to some external influence, it will be in the state of lowest energy (
ground state) in which the electrons systematically fill all the orbits from those nearest the nucleus outward to some larger orbit containing the outermost (valence) electrons. A valence electron...

lasers

...level for an individual atom occurs when its electrons are all in the nearest possible orbits to its nucleus. This condition is called the
ground state. When one or more of an atom’s electrons have absorbed energy, they can move to outer orbits, and the atom is then referred to as being “excited.” Excited states are...

photochemical reaction

...other molecules, or transfer electrons, hydrogen atoms, protons, or their electronic excitation energy to other molecules. Excited states are stronger acids and stronger reductants than the original
ground states.

...of optical radiation involves promotion of an electron from a low-energy orbital to a more energetic orbital. This is synonymous with saying that the molecule (or atom) is promoted from its
ground state (or lowest energy state) to an excited state (or higher energy state). This excited-state molecule often has drastically different properties from the ground-state molecule. In addition,...

...the Pauli exclusion principle. If every occupied (or electron-containing) orbital holds a pair of electrons with opposing spin, the molecule is in a singlet state, which is the pattern for the
ground state of most molecules. When the molecule is excited (e.g., by absorption of a photon), one electron is promoted to a previously unoccupied orbital, and, if its spin does not change, then the...

Photosensitized molecular oxygen is a powerfully oxidative species that severely hampers the photosynthetic efficiency of plants and causes health problems such as cataracts in humans. The
ground state of molecular oxygen is very unusual in that it is a triplet; hence, it can accept electronic energy from more-energetic triplet states of other molecules in a process called quenching (as in the...

...converts the central double bond from
trans to
cis. As in photodissociation, this is caused by the electron distribution in the excited state being quite different from that in the
ground state; hence, the structure of the initially created excited singlet (by absorption of light) is most stable at 90°, or halfway between the
cis and
trans forms. The molecule...

resonance photo-ionization

...selectivity is resonance ionization. In this scheme, a laser with adjustable wavelength irradiates the volume of gas from which the ions are to be extracted, exciting a transition from an atom’s
ground state to one of its excited (high-energy) states. This strong excitation enables an equilibrium to be established between the two states, while at the same time other radiation—or...